The Tricky Way We Define a Moon

In less than a week, the entirety of North America will experience a partial or total eclipse, where the sun will temporarily be blocked by the moon. Therefore, it's worth taking a moment to talk about what exactly a 'moon' is. Our moon is a moon pretty much by default, but what about objects around other planets?

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Minute Earth talks about why deciding what is and isn't a moon is harder than it sounds:

Space is a very big place, and there are things of all shapes and sizes. Some are very big, some are very small, and they orbit around each other in all sorts of combinations. Separating all of these objects into 'planets' and 'dwarf planets' and 'moons' is a difficult task, and almost any standard we use will ultimately fall short.

For instance, what if we define a moon as any natural satellite that orbits a planet? Seems pretty straightforward, but does that mean every grain of dust in Saturn's rings is a moon? Surely that can't be true, which means we need a size limit for moons.

Perhaps they need to be large enough to form spheres, like planets do. But such a limit would not only disqualify some of the smaller moons of Jupiter and Saturn, it would also exclude Mars' moons Phobos and Deimos, which are only a few miles wide. Are we okay with that?

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Or, take Pluto and Charon. Charon is commonly referred to as a moon of Pluto, but Charon is huge when compared to the dwarf planet. Pluto is less than twice as big as Charon, so it might be more accurate to refer to the two as a binary pair. Instead of Charon orbiting around Pluto, both orbit around a common center of mass that lies outside either object.

So where does that leave us? Perhaps with a working definition that isn't perfect, but might be good enough: a moon is a natural satellite of a planet or dwarf planet that's large enough to pull itself into a sphere and small enough so the center of mass lies within the planet or dwarf planet.

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